6 ME. E. D. OLDHAM ON THE INTEEPEETATION [Feb. 1909, 



From these figures it will be seen that the statements in Messrs. 

 Hay ford & Baldwin's report as to the greater movement near the 

 fault are borne out, although the figures are slightly different ; 

 excluding the stations on the west side of the fault in the San 

 Francisco group, the displacement in the outer zone is from 20 to 

 60 per cent, less than in the inner, and the decrease is more rapid 

 on the eastern than on the western side. Moreover, these displace- 

 ments are positive ; that is to say, the stations near the fault have 

 not lagged behind, but have moved forward in opposite directions 

 to a greater distance than those farther away. 



The increase of displacement was not, apparently, carried right 

 up to the fault-line, and the shifting along this seems to have been 

 less than the relative displacement of points at a little distance from 

 the fault. The published data do not admit of any direct comparison, 

 as we do not possess a series of measurements at points situated 

 along or near to a straight line at right-angles to the run of the 

 fault ; but the displacements at the fault-line, as measured by 

 the offset of interrupted fences and roads, seem to have been no 

 greater, and sometimes less, than those at a distance of a mile or so 

 away. 1 



The diminished displacement at the fault-line is not difficult to 

 explain, and may reasonably be ascribed to a frictional drag or 

 resistance to movement along this plane. The increase in displace- 

 ment as the fault is neared, and until it is actually reached, is not 

 so easy to explain ; but, before dealing with this point, it will be well 

 to see how far from the fault-line permanent displacements can be 

 recognized, and to determine the area which has suffered permanent 

 distortion in connexion with the earthquake. 



In the Eeport it is assumed that the stations Mocho and Mount 

 Diabolo had suffered no displacements, and that the line joining 

 them could be accepted as a base-line of the triangulation in 

 1906-07, being unaltered in length or direction. The observations 

 show that there has been no appreciable change in the azimuth of 

 the line joining these two stations, and the authors have most con- 

 clusively shown that the apparent displacement of other stations 

 cannot be explained by any alteration in its length ; but they have 

 not shown that there was no change in the length of the accepted 

 base-line, and it is necessary to consider whether the displacement 

 of stations, which certainly took place in connexion with this 

 earthquake, may not have affected the assumed unaltered base. 



In 1897, after the great earthquake in Assam, a part of the 

 primary triangulation in the disturbed area was reobserved, and the 

 result, as published by the Great Trigonometrical Survey, indicated 

 an apparent enlargement of the area resurveyed. In this case, it 

 was possible to show that the line which was taken as an unaltered 



1 [The Eeport of the State Earthquake Investigation Commission, received since 

 this paper was read, shows that this was confined to the immediate neigh- 

 bourhood of the fault-line. Fig. 15 on p. 64, fig. 21 on p. 71, and fig. 38 on 

 p. 101 are plans of dislocated fences which show clearly a drag or diminished 

 displacement within 120 feet or less of the dislocation.] 



